(i) Field of the Invention
This invention relates to a process for treating lignocellulosic pulp fibres of either softwoods or hardwoods to provide pulps of improved properties. In particular this invention is directed to the treatment of mechanical pulps and high-yield chemical pulps to improve and retain the properties of such pulps.
(ii) Description of the Prior Art
Newsprint traditionally has been manufactured from a furnish consisting of a mixture of a mechanical pulp and a chemical pulp. Mechanical pulp is used because it imparts certain desired properties to the furnish: namely, its high light scattering coefficient contributes to paper opacity and allows the use of a thinner sheet; its high oil absorbency improves ink acceptance during printing.
Chemical pulps are used because they impart properties to the furnish which improve its runnability. Runnability refers to properties which allow the wet web to be transported at high speed through the forming, pressing and drying sections of a papermachine and allows the dried paper sheet to be reeled and printed in an acceptable manner. Runnability contributes to papermachine and pressroom efficiency.
It is believed that improved runnability in chemical pulp is due to high wet-web strength and drainage rate. Wet and dry stretch are important because they are believed to contribute to preventing concentrations of stress around paper defects, thereby minimizing breaks. High drainage rates lower the water content and are believed to yield a less fragile web.
Mechanical pulps including stone groundwood (SG) and pressurized stone groundwood (PSG) can be made to provide wet stretch but only at the expense of poor drainage. Higher quality mechanical pulps are obtained by manufacture in open discharge refiners, to produce refiner mechanical pulp (RMP) and in pressurized thermomechanical pulp (TMP). Still further upgraded mechanical pulps were provided by chemical pretreatment of the wood chips prior to refining to provide chemimechanical pulp (CMP or CTMP).
U.S. Pat. No. 3,446,699 issued May 27, 1965 to Asplund et al. provided a method for producing mechanical and chemimechanical or semichemical pulps from lignocellulose-containing material, in order to provide what was alleged to be improved quality of the fibres with improved defibration.
U.S. Pat. No. 3,558,428 issued Jan. 26, 1971 to Asplund et al. provided a method for manufacturing chemimechanical pulps involving heating and defibrating the same in an atmosphere of vapour at elevated temperatures and under corresponding pressure of the impregnated chips to provide a more rapid and effective impregnation.
U.S. Pat. No. 4,116,758 issued Sept. 26, 1978 to M. J. Ford provided a process for producing high-yield chemimechanical pulps from woody lignocellulose material by treatment with an aqueous solution of a mixture of sulfite and bisulfite, to provide a pulp which can be readily defibered by customary mechanical means to provide a pulp having excellent strength characteristics.
Today's papermaker is faced with the problems of decreasing forest resources, an increasing demand for paper products and stringent environmental laws. Low-yield chemical pulps, e.g. sulphite and kraft pulps, contribute highly to such problems.
The fibres of low-yield chemical pulps are known for their desirable dry- and wet-web strength properties. Observations of low-yield chemical fibres in a formed paper sheet indicate that these tend to have a kink and curl which is said to contribute, in an advantageous way, to the papermachine runnability and to certain physical properties. Mechanical pulps lack the desirable strength properties to replace, in whole or in part, low-yield chemical pulps, e.g. kraft or sulphite pulps, in linerboard, newsprint, tissue, printing grades and coated-base grade of paper. Consequently, it has been an aim of the art to improve the physical properties of mechanical and high-yield chemical pulps, so that such improved pulps would be used to replace low-yield chemical pulps.
A number of mechanical devices have been built to produce curled chemical and mechanical fibres in order to improve certain physical properties. Two such mechanical fibre-curling devices are disclosed in H. S. Hill, U.S. Pat. No. 2,516,384 and E. F. Erikson U.S. Pat. No. 3,054,532.
H. S. Hill et al. in Tappi, Vol. 33, No. 1, pp. 36-44, 1950, described a "Curlator" designed to produce curled fibres. The process consisted of rolling fibres into bundles at a consistency of around 15%-35%, followed by dispersion. Advantages claimed were higher wet-web stretch, improved drainage, and higher tear strength and stretch of the finished product. These advantages were at the expense of certain other properties, notably tensile strength.
W. B. West in Tappi, Vol. 47, No. 6, pp. 313-317, 1964, describes high consistency disc refining to produce the same action.
D. H. Page in Pulp Paper Mag. Canada, Vol. 67, No. 1, pp. T2-12, 1966, showed that the curl introduced was both at a gross level and at a fine level which he called "microcompressions". Both types of curl were advantageous.
J. H. De Grace and D. H. Page in Tappi, Vol. 59, No. 7, pp. 98-101, 1976, showed that curl could be produced adventitiously during bleaching of pulps, by the mechanical action of pumps and stirrers at high consistency.
R. P. Kibblewhite and D. Brookes in Appita, Vol. 28, No. 4, pp. 227-231, 1975, claimed that this adventitious curl could have advantages for practical runnability of papermachines.
High-consistency mechanical defibration of wood chips is known to produce curled, kinked and twisted fibres. Kinked fibres are known to be particularly effective in developing extensibility in wet webs if the kinks are set in position so that they survive the action of pumps and agitators at low consistency and retain their kinked and curled state in the formed sheet. This ensures enhancement of the wet-web stretch and certain other physical properties.
A number of chemical treatment methods have been reported to enhance and retain fibre curl in a refined pulp. In one, Canadian Pat. No. 1,102,969 issued June 16, 1981 to A. J. Kerr et al., improvement in tearing strength of the pulp is alleged by the treatment of delignified lignocellulosic or cellulose pulp derived from a chemical, semichemical or chemimechanical pulping process at a pressure of at least one atmosphere, with sufficient gaseous ammonia to be taken up by moist pulp in an amount greater than 3% by weight to weight of oven dried pulp.
In another, Canadian Pat. No. 1,071,805 issued Feb. 19, 1980 to A. J. Barnet et al., a method of treatment of mechanical wood pulp is provided by cooking the pulp with aqueous sodium sulphite solution containing sufficient alkali to maintain a pH greater than about 3 during the cooking. The cooking was effected at an elevated temperature for a time sufficient to cause reaction with the pulp and to increase the drainage and wet stretch thereof, but for a time insufficient to cause substantial dissolution of liquor from the pulp, and insufficient to result in a pulp yield below about 90%. A minimum concentration of sodium sulphite was 1% since, below 1% sodium sulphite improvements were said to be too small to justify the expense of treatment.